1. Field
The present disclosure generally relates to packaging of semiconductor dice and, more particularly, to structures and methodology for reducing voids in solder used in the die attach process.
2. Description of the Related Art
Integrated circuit (IC) chips are enclosed in a variety of packages including dual inline pin (DIP) devices, plastic leaded chip carrier (PLCC) devices, and ball grid array (BGA) devices. Many of these contain multiple internal semiconductor components that are mounted to a substrate, sometime referred to as a “leadframe”, in a process called “die attach” or “die bond” and interconnected by a process such as wirebonding. The substrate is often metal and the components are soldered to the substrate to provide mechanical attachment and good thermal contact to dissipate heat generated in the component. The devices may also be overcoated or encapsulated with a plastic molding compound to provide protection during manufacturing and operation. The entire assembly of substrate, components, external connector pins or pads, and molding compound is then enclosed in a shell that may be, in many commercial applications, made of plastic.
Meeting the moisture resistance test (MRT) requirements is a key qualification process for any semiconductor package. A performance requirement set forth by the Joint Electronic Device Engineering Councils (JEDEC) addresses the effect of moisture on nonhermetic packaged solid state devices. Moisture can affect the substrate, molding compound, and die integrity. The most common failure during the MRT is delamination failure that occurs at any one of the package interfaces. In general, delamination in the package starts from the weakest interface and propagates outwards to the package edge or to other interfaces.
The challenge in maintaining integrity of the package is even greater when solder (a metal alloy) is used as the attach material for the various package components. Low-cost lead-free solder materials that are available in the market have melting temperatures that range from 217 degrees C. for Sn/Ag/Cu (SAC) based alloys to 250 degrees C. for Sn/Sb based alloys. Low-cost lead-based solders with melting temperatures of around 300 degrees C. and above are also available but rarely used in module packaging assembly due to the temperature limitations of passive components.
When solder is used to attach a semiconductor die, large voids can form within the solder during reflow due to failure of air to escape as the solder flows. Voids can also be created by the generation of gas from the flux or moisture present in the area. Large voids within the bulk of the solder may reduce the mechanical strength of the bond between the die and the substrate and can be points of crack initiation when the solder is under stress. The trapped gases within the voids in the solder can act as high vapor pressure points that worsen package moisture sensitivity level. Semiconductor packages must also be able to withstand surface mount temperatures in excess of 250 degrees C. during printed circuit board (PCB) assembly. Failure to meet the temperature requirements may tend to lead to component failure during board mount or, worse, field failures.
There are several reasons why molten solder can flow and then undesirably spread out within an encapsulated package. These reasons include mismatch between the coefficients of thermal expansion of the die and substrate materials, vapor pressure, temperature and moisture. When the critical stresses at the interfaces are exceeded, these factors tend to lead to unstable void growth, debonding and delamination at package interfaces. Because of the porous nature of the materials used in manufacturing electronic packages, the packages will tend to collect moisture when left exposed, unsealed, in an uncontrolled environment. Moisture in the package may vaporize and develop pressure when the temperature within the package is elevated. This pressurized gas will push the molten, non-porous solder out from under the die, possibly creating shorts between circuits in the package as well as reducing the strength of the bond of the die to the substrate. Secondly, moisture within electronic packages causes interfacial adhesion degradation at the interfaces (solder mask to copper, solder mask to mold compound, etc). Therefore, the package moisture sensitivity performance will largely be determined by the adhesion strength of the interfaces at elevated temperatures. Once the interface delaminates, the moisture can penetrate under the die and later push out the solder if the solder melts in a subsequent operation such as PCB reflow processing.
Molten solder can also be pushed out of the package by vapor pressure build-up in the voids present in the solder itself. The pressure in the voids can be high and, when no escape path for the gas is present, the void will expand within the bulk of the solder itself. Moisture that has penetrated along a degraded interface into the interior area of the solder may vaporize when the temperature rises, also leading to solder spread. In general, these two events are not exclusive and occur together to cause solder flow in the package, which leads to failure.
FIGS. 1A & 1B illustrate a typical current design for a die attach pad 10 before a die 108 is soldered in place. FIG. 1A shows a top view of substrate 100 with a solder mask 102 applied to the surface of the substrate 100. FIG. 1B shows a cross-section view taken at the location and in the direction indicated by dotted line 99 in FIG. 1A. An opening 104 is created in the solder mask 102 to define the die attachment area, also referred to as a die attach pad, to be wetted by the solder, where the opening 104 is slightly typically larger than the die 108 that will be attached, the die profile 120 indicating where the die edges will be when the die is attached.
FIGS. 2A & 2B illustrate a typical current design for a die attach pad, after a die is soldered in place. FIG. 2A shows a top view and FIG. 2B shows a cross-section view, taken at section 99 of FIG. 2A, of the same opening 104 after die 108 has been attached with solder 106. It can be seen that the die 108 is floating on the solder 106 and is raised above the solder mask 102. A typical solder mask is approximately 30 micrometers (1.2 mils) thick while a typical solder stencil (not shown), used to apply solder paste to select locations, is approximately 125 micrometers (5 mils) thick, leaving a layer of solder paste (not shown) that is much thicker than the solder mask 102. Some of this solder paste may be flux that will be removed during solder reflow but the remaining solder 106 will remain as a thicker layer than the surrounding solder mask 102. Distance L1 is the distance from the center of the die to the nearest edge of the die, which is the distance that a void, or bubble, formed at the center of the solder pad would have to travel to reach a free edge of the solder where the bubble could vent to the atmosphere. As L1 become larger, a higher pressure is required in the void to push the solder out of the way over the distance L1. A void 110 is shown in FIG. 2B that was formed during the attach process and unable to escape from the bulk material of solder 106 while the solder 106 was molten.
FIGS. 3 & 4 are x-ray photographs, taken through the body of a die 108 attached using the current design of the die attach pad, showing voids in the solder 106. FIG. 3 shows a large void 110 covering approximately 25% of the area of solder 106 with numerous other smaller voids 110 also visible. Similarly, FIG. 4 shows numerous small-to-medium voids 110 in solder 106. These voids may have remained in the solder because there was insufficient time while the solder is molten for the gas to reach the atmosphere at the edge of the die or that there was insufficient pressure in the voids to displace the amount of solder between the voids and the atmosphere. These are usually considered failures per die attach process inspection standards.
FIG. 5 is a composite of two photographs showing solder spread after reflow during a subsequent printed circuit board (PCB) assembly process from a die attached using the current design of the die attach pad. Die 108 was bonded with solder 106 and underwent a 260 degree C. PCB mount reflow. Solder spread 120 is solder that has been pushed out from under die 108 and come into contact with devices 125 and 130.
FIG. 6 is an enlarged cross-section of a typical current die attach pad design. The void 110 is separated from the nearest atmosphere by distance D1. The pressure in void 110 during the die attach process was insufficient to displace the amount of solder 106 present in distance D1 and therefore the void 110 remained in the bulk of solder 106 until the end of the die attach process. Also shown are two typical failures that may be related to void 110. Crack 310 can be created when pressure is formed in void 110 by heating during subsequent reflow or rework operations. As the pressure in void 110 presses upwards and downwards on the top and bottom interior surfaces, respectively, of void 110, tensile stress is created in the solder 106 at the tip 305 of void 110. If the tensile stress exceeds the tensile strength of solder 106, a crack 310 can form starting at tip 305 and propagating through solder 106. Pressure in void 110 will also press to the right and left, in this view, against the solder 106 and create compressive stress in the bulk material of solder 106 and in the adjacent solder mask 102. This may produce a failure where the solder mask 102 delaminates from the substrate 100 starting at corner 315. The solder 106 penetrates along the interface between the solder mask 102 and the substrate 100, creating a flow path 320 that can reach other devices or circuits.
FIG. 7 is an enlarged cross-section of a typical current die attach pad design, showing tilt of the attached die. FIG. 7 illustrates how a typical solder die attach pad, such as shown in FIGS. 2A & 2B, may allow an angle 350 of as much as five degrees (not shown to scale) relative to the substrate as the die 108 is floating unconstrained on the top of a single mass of solder 106.
Hence, solder used to attach semiconductor dice accordingly to the current processes is subject to formation of internal voids during reflow processes during assembly due to, among other causes, the presence of moisture. Moisture can also penetrate the semiconductor package and, after weakening one of more interfaces between components and materials within the package, migrate into the solder joint area. Subsequent heating of the solder joint, such as during a PCB assembly process, can vaporize this moisture, leading to solder spread and delamination. There is a need to provide a mechanism or process to minimize the formation of voids during and after the die attach process.